1. Field of the Invention
The present invention relates to an electronic control unit having an analog input signal that is improved so as to obtain a digital conversion output having high-precession and high-reliability by correcting a linear error of a digital conversion characteristic to an analog input signal of a multi-channel analog-digital converter (hereinafter referred to as AD converter).
2. Description of the Related Art
There is a limit to secure high digital conversion precision by analog calibration in a manufacturing process of AD converters, and thus there has been adopted a method of subjecting inexpensive and low-precision AD converts to digital calibration at a practical use stage to obtain required conversion precision. For example, “DIGITAL CALIBRATION METHOD AND APPARATUS FOR AD CONVERTER OR DA CONVERTER” described in JP-A-2000-295102 discloses, as a method of performing efficient calibration on AD converters, a digital calibration method for an AD converter which is characterized by comprising the steps of: inputting at least two different reference analog signals to an AD converter to be calibrated; calculating a correction parameter on the basis of each digital signal converted in the AD converter in accordance with the input of each reference analog signal; and executing correcting operation on the output digital signal of the AD converter by using the correction parameter calculated in the correction parameter calculating step.
According to the above publication, when a reading error occurs due to noise malfunction or the like in the step of inputting at least two different reference analog signals to the AD converter to be calibrated or when a calculation error occurs in the step of calculating the correction parameter on the basis of each digital signal converted in the AD converter in accordance with the input of each reference analog signal, a digital conversion value which is subsequently subjected to correction calculation by using an improper correction parameter continues to contain an abnormal error. A factor which induces occurrence of the calculation error resides in that even in the case of use of a high-precision reference analog signal, the converted digital value varies when a voltage applied to a reference voltage terminal of the AD converter varies. In the case of an AD converter which is used without being subjected to the correcting operation, an erroneous digital conversion value caused by a temporary noise malfunction is restored to a normal value by next measurements which are periodically read out (executed). However, if the correction parameter has abnormality, it induces a defect which is not temporary, but critical.